As is generally known in the art, a liquefied natural gas (sometimes referred to as an “LNG” hereinbelow) refers to colorless, transparent, cryogenic liquid obtained by cooling a natural gas mainly composed of methane to a temperature of −163° C. and reducing the volume thereof to about 1/600.
The LNG liquefied into a cryogenic state under an atmospheric pressure or a pressure higher than the atmospheric pressure is stored in a storage tank. The LNG thus stored is heated later and transformed into a gas phase, which process is generally referred to as a liquefied gas regasification process.
Conventionally, the task of regasifying the LNG has been performed on the land. As an LNG carrier arrives at a destination port, the LNG is transferred to land-based regasification facilities where the LNG is unloaded by an cryogenic pump and stored in a liquefied state or in a gaseous state.
However, it is difficult to install the dangerous regasification facilities on the land. Further, the regasification facilities pose a severe problem in safety because they are vulnerable to the threat of tenor.
In order to avoid this problem, it is the recent trend that the LNG is regasified on the sea by using an LNG regasification vessel (RV), a floating storage and regasification unit (FSRU) or the like.
Furthermore, it is the conventional method that a natural gas produced in a marine gas field is fed to a land-based liquefaction apparatus through a pipeline and is liquefied by the liquefaction apparatus. The liquefied natural gas is stored in a land storage house and is transferred to an LNG carrier by means of an cryogenic pump.
Along with development of small and medium size gas fields, it is often the case that the liquefied natural gas is regasified on the sea by using floaters such as a floating production storage off-loading (FPSO) unit and the like.
FIG. 1 schematically shows a process of unloading the gas regasified in a conventional LNG regasification vessel or a floating storage and regasification unit. FIG. 2 schematically illustrates an LNG loading process performed in a conventional floating production storage off-loading unit. Referring to FIG. 1, the liquefied natural gas stored in individual storage tanks 10 of a floating storage and regasification unit is regasified and unloaded to the land. In other words, the liquefied natural gas is discharged by pumps 12 arranged within the respective storage tanks 10 and is supplied to a regasification plant 16 through an upwardly-extending pipeline 14. The liquefied natural gas is regasified through a heatup process in the regasification plant 16 and is stably unloaded to the land through seabed pipelines by means of a submerged turret loading (STL) system arranged below the fore part of the floating storage and regasification unit.
Referring to FIG. 2, the natural gas produced in a gas field is loaded to the floating production storage off-loading unit. More specifically, the natural gas just produced in the gas field is introduced into the floating production storage off-loading unit by means of a submerged turret loading system. The natural gas thus introduced is liquefied by a liquefaction plant 24 into a cryogenic state and is loaded to individual storage tanks 20 through a pipeline 22 by means of cryogenic pumps 21.
Since the conventional floaters such as the LNG regasification vessel, the floating storage and regasification unit and the floating production storage off-loading unit are designed to load and unload the liquefied natural gas on the sea, they suffer from a serious drawback in that the storage tanks may be damaged by sloshing. In particular, most of the storage tanks are of a membrane type which is easy to increase the length and width thereof but is vulnerable to the sloshing.
The storage tanks of the floaters may be fabricated into self-supporting storage tanks of a SPB type (Self-supporting Prismatic-Shape IMO type B) or a Moss type that show an increased resistance against sloshing and external shocks. However, the SPB type storage tanks are expensive and, therefore, the LNG carrier becomes costly if the storage tanks are all fabricated into the SPB type. On the other hand, the Moss type storage tanks have no sufficient space for receiving a regasification apparatus and a liquefaction apparatus.